Treats the SDK and all application code as a legacy library when
running without enabling the null safety experiment.
- Copy all changes from the SDK fork and applies them to the existing
SDK (except the null safety syntax used in the library itself.
- Start emitting legacy types.
- Add a work around to avoid ever creating legacy or nullable versions
of types in the dart:_runtime library or in JS contexts. This should
be safely removed once the SDK is opted in to null safety.
Fixes: #40266
Change-Id: I6a029b676c3e35980a6bab876a348c35590df92d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/142904
Reviewed-by: Mark Zhou <markzipan@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Nicholas Shahan
8a6d0b57ea